Cellular phones (handsets) are increasing in complexity as handsets incorporate more types of service. In most of the world, the trend is for a handset to work with both Code Division Multiple Access (CDMA) type signals as well as the older Frequency Modulation (fm) type signals. These different modulation schemes both coexist in the same signal band and exist in different bands. The handset may have to transition from one to the other.
There are four major GSM standards as shown in Table 1. The low band contains two standards: GSM 850 and GSM 900. The high band also contains two standards: GSM 1800 and GSM 1900. GSM 850 operates in the United States, in the cellular band. GSM 900 operates in Europe. GSM 1800, also known as DCS, also operates in Europe. GSM 1900 operates in the United States in the PCS band. The modulation scheme WCDMA operates in most of the world (with the U.S. the major exception), in the UMTS band.
TABLE 1StandardTransmitReceiveGSM 850 824 to 849 MHz 869 to 894 MHzGSM 900 890 to 915 MHz 935 to 960 MHzGSM 18001710 to 1785 MHz1805 to 1880 MHzGSM 19001850 to 1910 MHz1930 to 1990 MHzUMTS1920 to 1980 MHz2110 to 2170 MHz
Global System for Mobile Communications (GSM) type phones are becoming the defacto global standard. Consequently, handsets that enable the GSM scheme must also support other schemes. To illustrate, 3rd Generation (3G) handsets must support both GSM and WCDMA. Both modes operate simultaneously, communicating in one mode and searching for service in another. Prior art architecture requires timing to avoid self-interference. This timing technique is called “Compressed Mode” (CM). In CM, the UMTS transmitter is temporarily switched into a state of twice the data rate, and thus twice the power. This allows for a following state where this transmitter is not broadcasting. The handset is then set to receive either GSM band, in order to establish a reliable link to a GSM basestation. This allows the handset to hand-over into the GSM network in a reliable manner. There is a price to be paid for this mode, however. First, this requires that the handset double it's output power. This is not possible if the handset is at the far end of it's range. Indeed, the most likely time for such a hand-over is just such a circumstance. Second, this increase in power results in a variable power at the basestation, impacting the network's power control. Loss of power control impacts the network capacity.
FIG. 1 shows a prior art Quad Band Dual Mode (QBDM) handset which requires compressed mode handover. Current 3G front-end circuits consist of two main components: the switch and the filters. The switch toggles between transmission (Tx) and reception (Rx) in either GSM mode or to the duplexer for the WCDMA in the UMTS band. The antenna switch determines which path is connected to the antenna. For GSM, either the transmitter or the receiver is engaged. For WCDMA, both Tx and Rx are on concurrently and must be kept distinct with a duplexer. In most of the world, the handset can operate in three modes: GSM 900, GSM 1800, or WCDMA. In the U.S., where the UMTS band is not allowed, the handset can operate in either GSM 850 or 1900.
FIG. 2 shows another prior art QBDM handset that supports compressed mode handover. Similar to FIG. 1, the switch toggles between transmission (Tx) and reception (Rx) in either GSM mode or to the duplexer for the WCDMA in the UMTS band. After the antenna receives the signal, the signal path is split into two by the diplexer. Positioned on either side of the diplexer are switches that further determine which path is connected to the antenna. For GSM, either the transmitter or the receiver is engaged. For WCDMA, both Tx and Rx are on concurrently and must be kept distinct with a duplexer. The handset can operate in three modes: GSM 900, GSM 1800, or WCDMA. While supporting non compressed mode in GSM 900, this architecture also supports only CM handover in GSM 1800. Similarly, there is no handover to the U.S. GSM bands.
FIG. 3 shows another prior art QBDM handset with compressed mode handover. This circuit requires careful design as the phase must be right for all of the filters. The handset supports non-compressed mode handover from WCDMA to only GSM 900. Handover to GSM 1800 requires compressed mode. In addition, there are four different states possible, one for each switch state. All present different impedances in the UMTS Rx band. The differing impedances make it difficult to properly tune the receiver, reducing sensitivity in some states, possibly increasing the number of frame errors. The handset itself counts upon the received signal to estimate the distance to the base station. The change in impedance will introduce error into this calculation, impacting the quality of the link.